The present invention relates to a starter system for a gas turbine engine, and more particularly to a control algorithm for an engine starter generator which operates to start a gas turbine engine.
In a conventional start system for a gas turbine engine, for example, an aircraft engine, a start sequence that coordinates engine speed, ignition and fuel delivery is required to achieve a reliable start. Conventionally, a dedicated starter motor or a starter-generator is driveably coupled to the gas turbine engine through a coupler shaft that drives the gas turbine engine when the starter generator system operates in a start mode and is driven by the gas turbine engine when the starter generator system operates in a generator mode. As the starter accelerates the engine, a fuel delivery pump provides fuel flow thereto. Igniters are then actuated to effect ignition in a combustor of the engine. Upon successful ignition, and once the engine has reached a self-sustaining speed, the starter is disengaged.
At the conclusion of the start cycle, the starter is transitioned to the generate mode under power, which may create a significant electrical transient to the power source that is supplying the electrical energy for the start. This transient may be problematic since the starter may not be supplied by a stiff electrical source for engine starts.
Furthermore, when the starter generator system transitions from Start mode to Generate mode, the electrical starter generator transitions from supplying mechanical power to absorbing mechanical power through the coupler shaft. This transition in power is accompanied by a change in the mechanical torque on the coupler shaft. This relatively sudden change in torque is effectively a step-function in which the torque is transmitted to the engine-mounted gearbox over a relatively short period of time. Such a torque transmission may result in undesirable mechanical load to the gearbox over prolonged periods of time.
Accordingly, it is desirable to provide a method of operating a starter generator system which minimizes electrical transients and the significant mechanical torque change applied to an engine gearbox of a gas turbine engine during the transition from a start mode to a generator mode.